It is often experienced that window glass of a vehicle is clouded due to dew condensation just after the starting of the drive in winter or in the rainy season. When men enter the vehicle and the temperature and the humidity in the vehicle rise rapidly, since the temperature of the metallic parts rises later than that of air in the vehicle because of the heat capacity thereof, dew condensation takes place also thereon just as on the surface of the window glass.
Recently R-DAT (Recordable Digital Audio Tape) has been developed and car audio devices employ R-DAT. Further, 8 mm VTRs (video tape recorders) for car are on sale and magnetic recording/reproducing apparatuses including a rotary magnetic head for the helical scanning method are used more and more widely as devices for a car.
However, since in a magnetic recording/reproducing apparatus of the helical scanning type, a tape is wound obliquely to a head drum rotating with a high speed and the head drum is rotated in contact to the magnetic tape with a relative speed of several m/sec in order to record information thereon, if dew condensation takes place on the rotating drum, the tape is stuck on the drum and the tape is drawn out in an instant. Thus the tape is disgorged in the neighborhood of the drum, as indicated in FIG. 11, or stuck around the drum, which can give rise in the worst case to an accident, because of which the cassette cannot be drawn out no more. In FIG. 11, reference numeral 1 is a drum; 2 is a S reel (supply side reel); 3 is a T reel (take-up reel); 4 is a capstan; and 5 is the disgorged tape. In general, in a home use VTR, the temperature is always supervised by a sensor and a heater is driven in a situation where dew is easily condensed, so that the neighborhood of the rotating drum is heated so as to prevent the appearance of the situation where dew is easily condensed. However, in the case where it is used for a car, in order to reduce the power consumption of the battery when the engine is not driven, it is necessary to keep the consumed current below several mA and therefore it is not possible to drive the heater without interruption as can be done for the home use VTR. Consequently, in the initial state, the temperature of the neighborhood of the rotating drum is the same as that of the ambient air. In addition, the room in the vehicle is almost closed and thus, when the temperature and the humidity rise rapidly as described previously, a situation, where dew is very easily condensed, appears. Since a certain period of time lapses before a dew condensation sensor detects the dew condensation state, under the rapid temperature and humidity rise, even if the operation of the system is adaped to stop by means of the dew condensation sensor, the accident described above can not surely prevent beforehand.
Further, even if the interior of the apparatus is not in the state where dew condensation takes place, when an extremely cold magnetic tape is brought in a warm room, it is conceivable also that dew condensation takes place on the tape itself and therefore it is impossible to prevent completely the sticking accident of the tape by using the dew condensation sensor disposed within the apparatus or a dehumidification measure.
Therefore, heretofore, in a case that the stoppage of the rotation of the reels, the capstans, the drum, etc. due to sticking of the tape in course of tape loading, recording/reproducing, etc. is detected, it is judged that the apparatus is not normal, an operation to power off the apparatus or to eject the cassette is effected. However, by this method the abnormality is detected, only when the tape is stuck really to the drum and it can rotate no more. Therefore, it was not possible to prevent beforehand the accident without impairing the tape.